1. Field of the Invention
The present invention relates to a magneto-optical recording medium such as a magneto-optical disk for writing and reproducing information with a laser beam or other similar devices.
2. Description of the Prior Art
The amount of data which must be dealt with in an information processing system is increasingly becoming larger, and a recording medium having a large storage capacity such as a magneto-optical recording medium has attracted attention. Magneto-optical recording is a type of vertical magnetic recording in which the direction of magnetization is vertical to a substrate plane, and upward and downward magnetization are assigned to signals of "0" and "1". Signals are recorded as domains which are heated above a Curie temperature to reverse the magnetization direction by an external field. The signals are reproduced by detecting the polarization of the laser beam reflected by the domains. According to the Kerr effect, a sign of a difference of the polarization between the incident and reflecting beams depends on the magnetization direction.
In order to increase the recording density of a magneto-optical recording medium, a super resolution technique is developed which can read signals even when a domain size is smaller than an optical resolution limit determined by a laser spot size (refer to U.S. Pat. No. 5,168,482). Thus even if two recording domains are included in a laser spot irradiated by the laser beam, information can be read.
The super resolution technique is divided into three types of detection: (a) Front aperture detection (FAD) where a recording domain is read from an area of lower temperature in a laser spot, (b) Rear aperture detection (RAD) where a recording domain is read from an area of higher temperature in a laser spot, and (c) Double aperture detection (DAD) which is a combination of FAD and RAD.
Magneto-optical recording media for super resolution has a structure different from an ordinary magneto-optical recording media. For example, a magneto-optical recording medium for front aperture detection comprises three magnetic layers, or specifically a recording layer, a switching layer and a readout layer. A magneto-optical recording medium for rear aperture detection comprises three magnetic layers, or specifically a recording layer, an intermediate layer and a readout layer. Information is recorded on the recording layer, and it is copied on to the readout layer during reproduction. Then, the signals are read by reading the direction of the magnetization in the readout layer.
The super resolution technique uses temperature distribution in a laser spot irradiated by the laser beam. However, the heat generated during erasing and recording causes a problem in that magnetic domains are affected by the heat generated. Even recording domains formed in a track adjacent to a particular track to be recorded or erased are likely affected by the heating. As the track pitch is made shorter in order to increase the recording density, this phenomenon is liable to occur more, especially in a land and groove recording where recording domains are formed along lands and grooves.
In a magneto-optical recording medium comprising layered magnetic layers, a half bubble may be generated in an adjacent track. In the half bubble, the magnetization in the readout layer is aligned along an applied external magnetic field, while the magnetization in the recording layer remains unchanged. In other words, a domain wall is also formed between the recording layer and the readout layer at the top of the half bubble. Because the above-mentioned domain wall also stores energy, the recording domain in the half bubble state may be more likely to vanish.
Further, if a magnetic field modulation for controlling the direction of applied magnetic field with a magnetic head is adopted for erasing and recording, a laser beam will continuously irradiate tracks. Therefore, the tracks are affected more by laser beam irradiation due to turning on or off the laser beam than the above-mentioned optical modulation for recording.
When the size of recording domains is decreased to obtain a higher recording density, the domains become easier to contract because a force for contracting a recording domain is inversely proportional to the size of recording domain. On the other hand, as a track pitch is shortened to obtain a higher recording density, the temperature of an adjacent track rises higher. Therefore, as the coercive force of the magnetic domain in the recording layer decreases with increasing temperature, an inhibition force for inhibiting movement of domain wall largely decreases, and magnetic domains become easier to contract.